Date:   Tue, 27 Sep 2022 22:23:37 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Pavel Begunkov <asml.silence@...il.com>, netdev@...r.kernel.org
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, linux-kernel@...r.kernel.org,
        xen-devel@...ts.xenproject.org, Wei Liu <wei.liu@...nel.org>,
        Paul Durrant <paul@....org>, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>
Subject: Re: [PATCH net-next 0/4] shrink struct ubuf_info

On Tue, 2022-09-27 at 21:17 +0100, Pavel Begunkov wrote:
> On 9/27/22 20:59, Paolo Abeni wrote:
> > On Tue, 2022-09-27 at 19:48 +0100, Pavel Begunkov wrote:
> > > On 9/27/22 18:56, Paolo Abeni wrote:
> > > > On Tue, 2022-09-27 at 18:16 +0100, Pavel Begunkov wrote:
> > > > > On 9/27/22 15:28, Pavel Begunkov wrote:
> > > > > > Hello Paolo,
> > > > > > 
> > > > > > On 9/27/22 14:49, Paolo Abeni wrote:
> > > > > > > Hello,
> > > > > > > 
> > > > > > > On Fri, 2022-09-23 at 17:39 +0100, Pavel Begunkov wrote:
> > > > > > > > struct ubuf_info is large but not all fields are needed for all
> > > > > > > > cases. We have limited space in io_uring for it and large ubuf_info
> > > > > > > > prevents some struct embedding, even though we use only a subset
> > > > > > > > of the fields. It's also not very clean trying to use this typeless
> > > > > > > > extra space.
> > > > > > > > 
> > > > > > > > Shrink struct ubuf_info to only necessary fields used in generic paths,
> > > > > > > > namely ->callback, ->refcnt and ->flags, which take only 16 bytes. And
> > > > > > > > make MSG_ZEROCOPY and some other users to embed it into a larger struct
> > > > > > > > ubuf_info_msgzc mimicking the former ubuf_info.
> > > > > > > > 
> > > > > > > > Note, xen/vhost may also have some cleaning on top by creating
> > > > > > > > new structs containing ubuf_info but with proper types.
> > > > > > > 
> > > > > > > That sounds a bit scaring to me. If I read correctly, every uarg user
> > > > > > > should check 'uarg->callback == msg_zerocopy_callback' before accessing
> > > > > > > any 'extend' fields.
> > > > > > 
> > > > > > Providers of ubuf_info access those fields via callbacks and so already
> > > > > > know the actual structure used. The net core, on the opposite, should
> > > > > > keep it encapsulated and not touch them at all.
> > > > > > 
> > > > > > The series lists all places where we use extended fields just on the
> > > > > > merit of stripping the structure of those fields and successfully
> > > > > > building it. The only user in net/ipv{4,6}/* is MSG_ZEROCOPY, which
> > > > > > again uses callbacks.
> > > > > > 
> > > > > > Sounds like the right direction for me. There is a couple of
> > > > > > places where it might get type safer, i.e. adding types instead
> > > > > > of void* in for struct tun_msg_ctl and getting rid of one macro
> > > > > > hiding types in xen. But seems more like TODO for later.
> > > > > > 
> > > > > > > AFAICS the current code sometimes don't do the
> > > > > > > explicit test because the condition is somewhat implied, which in turn
> > > > > > > is quite hard to track.
> > > > > > > 
> > > > > > > clearing uarg->zerocopy for the 'wrong' uarg was armless and undetected
> > > > > > > before this series, and after will trigger an oops..
> > > > > > 
> > > > > > And now we don't have this field at all to access, considering that
> > > > > > nobody blindly casts it.
> > > > > > 
> > > > > > > There is some noise due to uarg -> uarg_zc renaming which make the
> > > > > > > series harder to review. Have you considered instead keeping the old
> > > > > > > name and introducing a smaller 'struct ubuf_info_common'? the overall
> > > > > > > code should be mostly the same, but it will avoid the above mentioned
> > > > > > > noise.
> > > > > > 
> > > > > > I don't think there will be less noise this way, but let me try
> > > > > > and see if I can get rid of some churn.
> > > > > 
> > > > > It doesn't look any better for me
> > > > > 
> > > > > TL;DR; This series converts only 3 users: tap, xen and MSG_ZEROCOPY
> > > > > and doesn't touch core code. If we do ubuf_info_common though I'd need
> > > > > to convert lots of places in skbuff.c and multiple places across
> > > > > tcp/udp, which is much worse.
> > > > 
> > > > Uhmm... I underlook the fact we must preserve the current accessors for
> > > > the common fields.
> > > > 
> > > > I guess something like the following could do (completely untested,
> > > > hopefully should illustrate the idea):
> > > > 
> > > > struct ubuf_info {
> > > > 	struct_group_tagged(ubuf_info_common, common,
> > > > 		void (*callback)(struct sk_buff *, struct ubuf_info *,
> > > >                            bool zerocopy_success);
> > > > 		refcount_t refcnt;
> > > > 	        u8 flags;
> > > > 	);
> > > > 
> > > > 	union {
> > > >                   struct {
> > > >                           unsigned long desc;
> > > >                           void *ctx;
> > > >                   };
> > > >                   struct {
> > > >                           u32 id;
> > > >                           u16 len;
> > > >                           u16 zerocopy:1;
> > > >                           u32 bytelen;
> > > >                   };
> > > >           };
> > > > 
> > > >           struct mmpin {
> > > >                   struct user_struct *user;
> > > >                   unsigned int num_pg;
> > > >           } mmp;
> > > > };
> > > > 
> > > > Then you should be able to:
> > > > - access ubuf_info->callback,
> > > > - access the same field via ubuf_info->common.callback
> > > > - declare variables as 'struct ubuf_info_commom' with appropriate
> > > > contents.
> > > > 
> > > > WDYT?
> > > 
> > > Interesting, I didn't think about struct_group, this would
> > > let to split patches better and would limit non-core changes.
> > > But if the plan is to convert the core helpers to
> > > ubuf_info_common, than I think it's still messier than changing
> > > ubuf providers only.
> > > 
> > > I can do the exercise, but I don't really see what is the goal.
> > > Let me ask this, if we forget for a second how diffs look,
> > > do you care about which pair is going to be in the end?
> > 
> > Uhm... I proposed this initially with the goal of remove non fuctional
> > changes from a patch that was hard to digest for me (4/4). So it's
> > about diffstat to me ;)
> 
> Ah, got it
> 
> > On the flip side the change suggested would probably not be as
> > straighforward as I would hope for.
> > 
> > > ubuf_info_common/ubuf_info vs ubuf_info/ubuf_info_msgzc?
> > 
> > The specific names used are not much relevant.
> > 
> > > Are there you concerned about naming or is there more to it?
> > 
> > I feel like this series is potentially dangerous, but I could not spot
> > bugs into the code. I would have felt more relaxed eariler in the devel
> > cycle.
> 
> union {
> 	struct {
> 		unsigned long desc;
> 		void *ctx;
> 	};
> 	struct {
> 		u32 id;
> 		u16 len;
> 		u16 zerocopy:1;
> 		u32 bytelen;
> 	};
> };
> 
> 
> btw, nobody would frivolously change ->zerocopy anyway as it's
> in a union. Even without the series we're absolutely screwed
> if someone does that. If anything it adds a way to get rid of it:
> 
> 1) Make vhost and xen use their own structures with right types.
> 2) kill unused struct {ctx, desc} for MSG_ZEROCOPY

Ok, the above sounds reasonable. Additionally I've spent the last
surviving neuron on my side to on this series, and it looks sane, so...

Acked-by: Paolo Abeni <pabeni@...hat.com>

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